1. (Field of the Invention)
The present invention relates to steel cords to be used as a reinforcing material for rubber articles such as pneumatic tires or industrial belts. The invention also relates to pneumatic tires using such steel cords in belt layers.
2. (Related Art Statement)
In a rubber article reinforced with steel cords, there is a problem in that steel filaments are corroded with water invading the rubber article to lower service life of the product.
For example, if the steel cord used in a belt of a tire has a cavity in it, the following problem will occur. That is, when a tread of the tire undergoes an exterior cut reaching the belt, water invades the belt, and spreads inside the cavity of the cord in a longitudinal direction of the steel cord. Consequently, rust resulting from the invading water spreads so that adhesion between the rubber and steel cord decreases at a rust-spread portion to produce a separation phenomenon.
Under the circumstances, in order to prevent cut separation due to propagation of the corrosion, a cord construction is proposed, in which rubber can fully penetrate into the cord through gaps among adjacent metallic filaments during vulcanization under pressure.
Japanese patent application Laid-open No. 63-235,587 discloses a pneumatic tire provided with a belt layer burying, in rubber, steel cords which have a 2+7 cord construction and a tenacity of 1,700 to 2,050d.sup.1.872 kgf/cord (d (diameter of filament)=0.28 to 0.40 mm).
Bridgestone Corporation formerly disclosed a steel cord having a 2+8 cord construction, in which a diameter of each filament is in a range of 0.30 to 0.42 mm, a difference (Dc-Ds) diameter of filament between a core and a sheath is not less than 0.4 mm, and the twisting direction of the core is the same as that of the sheath, and a pitch ratio Ps/Pc between the core and the sheath is 1.4 to 3.0, as well as pneumatic tires using such cords in a belt layer (Japanese patent application Laid-open No. 2-154,086. In this tire, cut separation of the tire is to be prevented by using rubber-permeable type steel cords in the belt layer.
However, steel cords disclosed in Japanese patent application Laid-open Nos. 63-235,587 and 2-154,086 are not optimized in terms of "forming", so that sheath-sheath gaps great enough to allow rubber to penetrate into the cords by vulcanization under pressure cannot be realized or restraint forces for mutual filaments in the core or sheath become insufficient to cause deviation in twisting filaments of the sheath, which forms a portion where the filaments of the sheath adheres to each other to hinder penetration of rubber into the cord through there (FIG. 5). Because of this, in the case of a pneumatic radial tire using such cords in a belt layer, that portion of the cord into which rubber does not sufficiently penetrate may appear even after vulcanization under pressure. Thus, it is not always guaranteed that cut separation resistance can be stably obtained as one of product performances for the tire.